Relative utilization of isoacceptor aa-tRNAs in protein synthesis have demonstrated variations in the incorporation of the corresponding amino acid into protein even though the isoacceptors have the same codon recognition properties. Two minor Ser-tRNAs which recognize specifically the nonsense codon UGA have been shown to be conserved in higher vertebrates. Patterns in codon recognition of isoacceptor aa-tRNAs from Physarum polycephalum have been determined and the aminoacylation of 19 aa-tRNAs fractionated from several species under homologous and heterologous conditions have been examined.